The present invention refers to a vehicle brake disc or a vehicle coupling or clutch disc having a carbon--carbon composite body and a silicon carbide coating, and to a method of manufacturing such a disc.
In the field of vehicle construction, carbon--carbon composites ("C--C composites") have been used up to now only for structural components of high-performance racing cars which are subjected to high temperatures. C--C composites are a special type of composite material which resembles carbon fibre-reinforced plastic materials to a certain extent. In contrast to these plastic materials, C--C composites consist of a carbon fibre substrate which is embedded in a carbon matrix.
The development of these materials has been started approximately in 1958 by the American air force for aeroplanes and by NASA. It was in connection with the space shuttle that these materials were used on a large scale for the first time.
Although both components of C--C composites, i.e. the fibre component and the filler component, consist of carbon, the behaviour of the material essentially depends on the state of the carbon of the respective component. Crystalline carbon, i.e. graphite, consists of closely joined hexagonal crystals which are arranged in layers and which are held together by Van der Waals forces. In addition to its defined forms, graphite and diamond, carbon can assume a large number of intermediate states in a quasi-crystalline form, from an amorphous, vitreous carbon to a highly crystalline graphite. The anisotropy of the individual carbon crystal permits a large number of combinations between the two carbon components of the composite.
For the fibre component, threads, bands or fabrics are used. The highest strength is achieved by a straight orientation of the fibres. For most technical applications, fabrics are, however, used, said fabrics being normally two-dimensional (2-D) fabrics. If a high strength in all three directions of space is required, it is also possible to use fabrics that are woven in three directions of space, i.e. 3-D fabrics.
The materials used as a matrix material are thermoplastic materials, pitch, phenolic resins and gaseous hydrocarbons.
One method of manufacturing C--C composites comprises the steps of modelling the intended workpiece from fibre material and filling its pores with carbon. For this purpose, the preformed workpiece is introduced in a furnace and exposed to a hyrocarbon gas, normally methane. In the course of this process, the carbon deposits in the fabric. At temperatures in the order of 1,100.degree. C., the deposited carbon has an isotropic structure and is referred to as pyrolytic carbon. Between 1,000.degree. C. and 1,700.degree. C., the deposited carbon has an intermediate-state microstructure which becomes increasingly graphitic as the temperature increases. At deposition temperatures between 1,700.degree. C. and 2,300.degree. C., graphite is deposited, said graphite being also referred to as pyrolytic graphite.
This process, which is also referred to as CVD (chemical vapour deposition) process in the literature, is very slow and necessitates that the process parameters are adjusted with high accuracy. Normally, it takes several weeks to finish a single part.
A second manufacturing method comprises the steps of hardening carbon fibre-reinforced ploymer structures and of converting this material then into the carbon state in an inert atmosphere at temperatures of approx. 850.degree. C. to 1,000.degree. C. The heating phase for converting the material into the carbon state is normally approx. 1 week. Pure C--C composites produced according to this method are already used in the production of brakes.
A C--C composite of higher density (p&gt;1.95 g/cm.sup.3) is obtained by a high temperature/high pressure process in the case of which a fabric is first impregnated with a highly carbonaceous material and is then converted into the carbon state at high temperatures. Although this method permits a great variety of microstructures, it is used very rarely because of the equipment and the safety measures required for generating the high pressure.
In contrast to metal and ceramic materials, C--C composites do not lose their strength even at high temperatures. High thermal conductivity and a low coefficient of thermal expansion make C--C composites resistant to strong temperature variations. In spite of their excellent high-temperature properties, C--C composites have only been used for brakes of aeroplanes and racing cars as well as for heat barriers of spacecraft and for rocket jets so far. A more frequent use of these materials has hitherto been limited by the fact that, in addition to the extremely high manufacturing costs, these materials begin to oxidize at temperatures of approx. 400.degree. C. For the above-mentioned former cases of use, this circumstance was unproblematic in view of the fact that the materials were always used for a limited period of time, but it prevented a long-term use of said materials, e.g. in standard type cars.
Although it has already been suggested to coat C--C composites so as to improve the oxidation behaviour, an acceptable composite system permitting e.g. the use of C--C composites in internal combustion engines has not been found so far. One of the main difficulties arising in connection with the coating is that damage, e.g. cracks in the coating, is caused due to the different thermal expansion behaviours of the C--C composite and of the coating material. Up to now, only approx. 10 to 15% of all C--C composites are actually protected against oxidation.
Up to now, silicon ceramics have primarily been used for the coating; these silicon ceramics permit, however, only a thermal load of the workpieces of less than 1,700.degree. C. The use of a coated C--C composite for heat barriers is known from space shuttles. In this case, silicon was diffused into intermediate-state carbon in an inert atmosphere at 1,760.degree. C. Subsequently, the material was impregnated with tetraethylorthosilicate (TEOS), hydrolyzed and heat-treated, whereby an silicon dioxide ("SiO.sub.2 ") coating having a thickness of approx. 0.5 mm was produced. This method is, however, very complicated from the technical point of view so that these materials are not suitable for use in the case of mass-produced parts. The same applies to the use of CVD processes for applying silicon carbide ("SiC") or silicon nitride ("Si.sub.3 N.sub.4 ") coatings to intermediate-state C--C composites.
In racing cars, only pure, uncoated C--C composites have, in practice, been used for the brakes up to now. On the one hand, the friction coefficient of such brakes at room temperature is not sufficiently high for permitting a satisfactory braking behaviour. On the other hand, an increased amount of oxidation occurs at high temperatures so that the operating range of the hitherto used C--C vehicle brake discs is between 400.degree. C. and 600.degree. C. It is therefore necessary to warm the vehicle brakes up prior to use, i.e. to drive a warm-up lap with a racing car. Hence, such brakes have not been suitable for use in standard type cars up to now, or they would require a complicate temperature control. Due to the high abrasion behaviour, the brake discs, moreover, wear very rapidly and must normally be exchanged after one race, consequently. The statements made hereinbefore apply, mutatis mutandis, also to clutch discs.
A brake disc of the type mentioned at the beginning is known from JP-A-5 059 350. The coating of said brake disc is, however, produced in that a C--C workpiece is heated in a poisonous silicon oxide ("SiO") atmosphere to temperatures of 1,300.degree. to 2,3000 so that an SiC layer is produced on the surface, or the whole workpiece is converted into a C--C/SiC composite. The Sic layer produced in this way is a diffusion layer. This is evident from the fact that the concentration of the silicon content decreases from the surface of the workpiece to the interior thereof only gradually, this being shown, by way of example, on the basis of curve "a" in FIG. 4. SiC in the interior of a workpiece is, however, undesirable in view of the poor thermal conductivity of SiC. In addition, the mechanical properties of the C--C composite within the brake disc are impaired.
The manufacturing method known from JP-A-5 059 350 is also disadvantageous insofar as the diffusion of SiC takes place during the conversion of the structure of the C--C workpiece. During the conversion process major dimensional changes occur, which can only be compensated for at the finished workpiece in the case of said JP-A-5 059 350. Due to the accuracy required with regard to the parallelism of the braking surfaces and the high hardness of SiC, this necessitates very complicated finishing processes.
Further C--C workpieces having an SiC surface layer are known from DE-A1-26 53 665 and EP-A1-0 300 756. In both cases, said Sic surface layers are diffusion layers having the above-mentioned drawbacks. In view of fact that diffusion processes require a long period of time, the above-mentioned methods are very slow and not suitable for series production.